Field of the Invention
This invention relates to the manufacture of rackets, and particularly tennis rackets although the principles thereof are applicable to any type of strung racquet, e.g. squash rackets, racketball rackets, and the like.
In recent years a substantial trend has developed towards the use of racket frames made of fiber reenforced resin or metal as opposed to the long standing use of rackets made of wood. Both metal and fiber reenforced resin rackets possess certain advantages over wooden rackets, for example, in that they are less subject to warping and fatigue. However, the metal rackets present further problems in that the equipment required to form such rackets is relatively expensive, they are subject to cracking wherever any welds are used in the racket, and the physical properties of density, strength and stiffness tend to result in a racket which is not sufficiently flexible and is difficult to obtain proper playing qualities.
The fiber reenforced resin rackets have a very high tensile strength, a good modulus of elasticity, the materials are relatively inexpensive, and they can be readily formed and otherwise handled. There are generally two ways of forming fiber reenforced resin rackets. The first is through utilization of a solid core about which a series of preimpregnated fiber strips are wound in a plurality of layers and then placed in a mold for curing so that the core and the layers of reenforced resin become essentially integral. Such a method is disclosed, for example, in U.S. Pat. No. 4,128,963. A second method of forming reenforced resin rackets is by utilization of a removable core about which the series of reenforced strips of impregnated fiber are wound and then molded, with the core being removed after the racket has hardened in its final form. Such a method is disclosed, for example, in the above referred to application Ser. No. 827,670.
The years of playing with these types of fiber reenforced resin rackets have indicated some problems in the area of formation of the holes for the strings in the head portion of the racket in that the rackets tend to be weaker in the areas where the holes are formed, and have shown tendencies to crack or break through these holes. In addition, the constant movement of the strings within these holes has resulted in a wearing away of the resinous material which results in the fairly strong fibers cutting through the strings and causing a continually increasing breakage of the strings as the racket continues to wear. Both of these problems are the result of the manner in which the holes are formed in the rackets.
In the past it has been common to form such holes by drilling after the racket has been completely formed and molded. This necessitates drilling through the fiber reenforcing as well as the resin which obviously tends to decrease the strength of the racket in the area of the holes that have been drilled, and also presents a plurality of frayed ends of the reenforcing fibers where they have been cut through which then, as the resin is worn away through use, tend to cut through the strings causing increasing breakage thereof.